Ceramic composites and process for manufacturing the same

ABSTRACT

The ceramic composites consisting of sintered alumina comprising a polycrystalline alumina matrix having a grain size of from 0.5 to 10 μm, and fine particles of TiB 2  2 μm or less in diameter being dispersed in the alumina grains, the composite alumina ceramic containing from 15 to 40% by volume of TiB 2  ; or a ceramic composites comprising a polycrystalline alumina matrix as above, fine particles of TiB 2 , and fine particles of SiC, the fine particles of TiB 2  and SiC being each 2 μm or less in diameter and being dispersed in the alumina grains, the ceramic composites containing from 5 to 30% by volume of TiB 2  and from 5 to 30% by volume of SiC.

FIELD OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to a high strength ceramic composites anda process for manufacturing the same. More specifically, it relates tohigh strength ceramic composites having excellent heat and wearresistances, yet capable of being machined by an EDM (electric dischargemachining) process and suitable as structural materials. The presentinvention relates also to a process for manufacturing the same.

Alumina (aluminum oxide, Al₂ O₃) has been long used as an industrialmaterial for a wide variety of applications because of its readyavailability by sintering and of its superiority in, for example,refractoriness, corrosion resistance, and electric insulatingproperties. However, it is still inapplicable to structural materialsbecause of its insufficient mechanical strength such as bendingstrength, fracture toughness, and resistance against thermal shock.Thus, as an approach to overcome those disadvantages, some of the R & Defforts have been paid on making composites thereof.

Those studies on manufacturing composites, however, were mostlyconcerned with how to achieve complexing in a micrometer scale, andhence the improvement of the mechanical properties was somewhat limited.It is generally believed that in a grain-dispersed composite, the cracksare deflected by the dispersed grains which are localized at the aluminagrain boundaries, and that the toughness of the resulting sinteredalumina is thereby increased.

In a ceramic sintering such as of alumina, the matrix thereof consistsof anisotropic grains. Accordingly, the localized stresses generate atthe grain boundaries by the thermal expansion mismatch between thematrix and dispersion : the localized compressive stress is accumulatedduring cooling down from the sintering temperature. Then, this grainboundary becomes a fracture source which decreases the strength of thewhole sintering.

When fine particles are dispersed in a matrix, it is expected that thetoughness is improved, because those particles avoid propagation of thecracks. However, a prior art technology that comprises dispersing thosefine particles in the matrix was by no means effective in significantlyincreasing the strength, because there was no change in the grainboundaries which function as the source for fractures.

OBJECT AND SUMMARY OF THE INVENTION

An object of the present invention is to overcome the prior art problemsmentioned hereinbefore, and to provide a high strength (refractory)ceramic composites and a process for manufacturing the same.

An another object of the present invention is to provide, by controllingmicrostructures, ceramic composites having considerably improved inthermal shock fracture resistances, fracture strength and fracturetoughness, thereby making it suitable for use as refractories,wear-resistant materials, cutting tool materials, corrosion-resistantmaterials, and the like. It is still another object to provide a processfor manufacturing the same.

The ceramic composites according to an embodiment of the presentinvention an characterized by that it comprises an alumina (Al₂ O₃)matrix comprising crystal grains from 0.5 to 10 μm in diameter, thealumina crystal grains having dispersed therein from 15 to 40% by volumeof fine titanium diboride (TiB₂) particles 2 μm or less in particlesize.

The ceramic composites according to another embodiment of the presentinvention are characterized by that it comprises an alumina (Al₂ O₃)matrix comprising crystal grains from 0.5 to 10 μm in particle size, thealumina crystal grains having dispersed therein from 15 to 30% by volumeof fine titanium diboride (TiB₂) particles 2 μm or less in particle sizeand from 5 to 30% by volume of fine silicon carbide (SiC) particles 2 μmor less in particle size.

The process for producing the ceramic composites according to anembodiment of the present invention comprises mixing fine alumina (Al₂O₃) particles 5 μm or less in diameter with from 15 to 40% by volume offine titanium diboride (TiB₂) particles 2 μm or less in diameter, andthen sintering the molding obtained from the resulting mixture at asintering temperature of 1400° C. or higher.

The process for producing the ceramic composites according to anotherembodiment of the present invention comprises mixing fine alumina (Al₂O₃) particles 5 μm or less in particle size with from 5 to 30% by volumeof each of fine titanium diboride (TiB₂) particles 2 μm or less inparticle size and fine silicon carbide (SiC) particles 2 μm or less inparticle size, and then sintering the molding obtained from theresulting mixture at a sintering temperature of 1500° C. or higher.

The present invention is characterized by that it provides a ceramiccomposite having improved in mechanical strength. This improvement wasachieved by realizing a material comprising fine particles of TiB₂ orfine particles of both TiB₂ and SiC having dispersed inside the grainsof alumina ceramics in a nanometer scale (i.e., a material composed ofcrystal grains being complexed in the level of minimum structural unitof a ceramic sintering). In such a sintered alumina comprising aluminacrystal grains having dispersed therein fine grains of TiB₂ or SiC, theresidual stresses generate between the alumina and those dispersed fineparticles ascribed to the difference in thermal expansion coefficient.Accordingly, compressive stresses generate at the grain boundary betweenthe neighboring alumina grains. Thus, the cracks inside the sinteringare pinned or sealed, or even deflected by those compressive strains.This is the believed mechanism which prevents propagation of cracksinside the sintering.

In an embodiment according to the present invention, only fine particlesof TiB₂ are dispersed inside the alumina grains. In another embodimentaccording to the present invention, fine particles of both TiB₂ and SiCare dispersed inside the alumina grains. It is characterized by that thesintered alumina is composed of alumina grains from 0.5 to 10 μm indiameter, and comprises dispersed therein TiB₂ and SiC grains 2 μm orless in particle size. It is characterized also by that the fine TiB₂particles and optionally the fine SiC particles are homogeneouslydispersed within the alumina matrix.

The ceramic composites according to the present invention ismanufactured by a process which comprises mixing alumina powder composedof grains 5 μm or less in particle size with fine TiB₂ particles andoptionally fine SiC grains both 2 μm or less in particle size, and thensintering a molding obtained from the resulting mixture.

To considerably increase the strength of the resulting sintering, it isrequired that the alumina matrix of the ceramic composites is controlledso that it may be composed of grains in the diameter range of from 0.5to 10 μm. Thus, by then incorporating particles of TiB₂ and SiC as fineas 2 μm or less in particle size, those fine particles are readilyincluded into the crystal grains of the alumina matrix during thesintering step.

The starting alumina powder should be composed of grains 5 μm or less indiameter. This enable ready sintering of the alumina grains. The fineparticles of TiB₂ and SiC should be 2 μm or less in diameter. By thuscontrolling the size of those fine particles to this specified sizerange, they are easily incorporated into the alumina grains withoutgenerating microcracks in the alumina grains.

In the case of dispersing fine TiB₂ particles alone, amount of itsaddition is from 15 to 40% by volume. An addition of the fine TiB₂particles within this specified range is effective for suppressing graingrowth during the sintering, which, as a result provides an increasedfracture strength and fracture toughness to the sintering ascribed tothe formation of fine sintered microstructures.

When fine particles of both TiB₂ and SiC are added, they are added at anamount of from 5 to 30% by volume each. The addition of the fine TiB₂and SiC particles within this specified range is effective forsuppressing grain growth during the sintering, which, as a resultprovides an increased fracture strength and fracture toughness to thesintering ascribed to the formation of a fine sintered microstructure.Furthermore in this case, it is preferred that the addition of fine TiB₂and SiC particles in total is in the range of from 10 to 40% by volume.

The fine TiB₂ and SiC powders for use as the starting material are thoseavailable by an industrial process.

It is require in the process for manufacturing the ceramic compositesaccording to the present invention, that the alumina grains are denselysintered during the sintering step, and that those alumina grainscontain homogeneously dispersed inclusions of fine TiB₂ or SiC particlestherein. Suitable processes for sintering the ceramic compositesaccording to the present invention include metallurgical techniques, hotpressing, pressureless sintering, and HIPing (Hot Isostatic Pressing)techniques. The sintering is carried out at 1500° C. or higher, andpreferably in the temperature range of from 1500° to 1800° C.

PREFERRED EMBODIMENTS

The ceramic composites according to the present invention and theprocess for producing thereof are described in further detail withreference to non-limiting examples. However, it should be understoodthat the present invention is not to be construed as being limited tothose examples.

EXAMPLES 1 TO 4, COMPARATIVE EXAMPLES 1 TO 4

The starting alumina powder was a commercially available powder, AKP-30(trade name for an α-alumina powder composed of grains 0.4 μm in averagediameter; a product of Sumitomo Chemical Company, Limited), and thestarting TiB₂ powder was TiB₂ -PF (a TiB₂ powder composed of grains 1.5μm or less in average diameter; a product of Japan New Metal, Ltd.)which was classified to obtain grains 0.6 μm or less in diameter.

Seven powder mixtures were prepared by adding the thus classified TiB₂powder with respect to alumina, at an amount by volume of 0%, 5%, 10%,15%, 20%, 30%, and 40%, in correspondence to Comparative Example 1,Comparative Example 2, Comparative Example 3, Example 1, Example 2,Example 3, and Example 4, respectively. Each of the powder mixtures thusobtained was subjected to conventional ball milling technique in thepure ethanol with alumina balls for 24 hours. The slurry thus obtainedwas sufficiently dried, and was crushed by dry ball milling for 12 hoursagain. Thus was obtained the powder mixtures.

Then, about 48 g each of the resulting powder mixtures was charged intoa graphite die (60 mm in inner diameter), and after subjecting themixtures to precompression applied pressure of 10 MPa, the resultingcompact was sintered using an frequency induction heating hot pressingapparatus (manufactured by Fuji Microwave Co., Ltd.) at temperaturesshown in Table 1. The hot pressing was conducted by first elevating thetemperature to a predetermined sintering temperature, and then holdingthe temperature constant for an hour in Argon gas atmosphere. Thepressure applied to the molding was 30 MPa.

The resulting sintered bodies of ceramic composites were each cut intotest pieces each at a specified dimension of 3×4×36 mm, according to theJIS R1601 standard three-point bending test. The bending strength wasthus obtained on those test pieces at a span length of 30 mm andapplying load at a crosshead speed of 0.5 mm/minute. The fracturetoughness was measured by the IF (Indentation Fracture) method withindenter loads of 5 kg for a retention time of 10 seconds. The size ofthe constituent grains was measured by means of electron microscopy. Theresults are given in Table 1.

                  TABLE 1                                                         ______________________________________                                        Bending strength for Al.sub.2 O.sub.3 --TiB.sub.2 composites                  varying in TiB.sub.2 addition                                                       Blending ratio                                                                            Sintering Grain size                                                                              Bending                                 Sample                                                                              (vol. %)    Tempera-  (μm)   Strength                                No.   Al.sub.2 O.sub.3                                                                      TiB.sub.2                                                                             ture (°C.)                                                                     Al.sub.2 O.sub.3                                                                    TiB.sub.2                                                                           (MPa)                               ______________________________________                                        Comp. 100      0      1400    <5    --    400                                 Ex. 1                                                                         Comp. 95       5      1500    <4    <0.6  500                                 Ex. 2                                                                         Comp. 90      10      1500    <3    <0.6  600                                 Ex. 3                                                                         Ex. 1 85      15      1550    <3    <0.6  800                                 Ex. 2 80      20      1600    <2    <0.6  950                                 Ex. 3 70      30      1600    <2    <0.6  1050                                Ex. 4 60      40      1650    <1.5  <0.6  900                                 ______________________________________                                    

It can be seen clearly from Table 1 that the bending strength of Al₂ O₃-TiB₂ composites to improves by adding TiB₂ in the range of from 15 to40% by volume. The fracture surface after breakage was observed on eachof the specimens having subjected to testing. Fracture surface wasobserved to be confined intergranular fracture because of the fine TiB₂particles homogeneously dispersed within the alumina grains. Thefracture plane obtained after the measurement of the fracture toughnesswas polished, and the cracks in this plane was observed. The cracks wereobserved to be deflected by the relatively coarse TiB₂ particles havingdispersed in the grain boundary, and to propagate along a verycomplicated path. Accordingly, it can be understood that the fracturestrength of the ceramic composites according to the present invention isimproved by the synergetic effect resulting from the addition of TiB₂particles which provide finely divided texture and change in thefracture mechanism, as well as the deflection of the crack propagationwhich increases the fracture toughness.

Further, the high temperature strength of Al₂ O₃ -TiB₂ compositesobtained in Example 2 above and containing 20% by volume TiB₂ wasmeasured. The results are given in Table 2.

                  TABLE 2                                                         ______________________________________                                        High temperature bending strength of                                          20 vol % TiB.sub.2 --Al.sub.2 O.sub.3 composites                              ______________________________________                                        Temp. (°C.)                                                                      200     400    600    800 1000 1200 1300                            Bending   940     940    960   1000 1050  900  520                            Strength (MPa)                                                                ______________________________________                                    

Table 2 reads that Al₂ O₃ -TiB₂ composites according to the presentinvention maintain the bending strength at the room temperature to atemperature as high as 1200° C. This is probably ascribed to thedispersed TiB₂ particles which play the role of suppressing the slips atthe grain boundary. Furthermore, upon observation of the fracturesurface, the fracture mechanism was seen to be an intergranularfracture, the same as that for the fracture occurring at the roomtemperature.

EXAMPLES 5 TO 8, COMPARATIVE EXAMPLE 4

The starting alumina powder was a commercially available powder AKP-30(trade name for an α-alumina powder composed of grains 0.4 μm in averagediameter; a product of Sumitomo Chemical Company, Limited). The startingTiB₂ powder was TiB₂ -PF (a TiB₂ powder composed of grains 1.5 μm orless in average particle size; a product of Japan New Metal, Ltd.) whichwas classified to obtain grains 0.6 μm or less in particle size, and thestarting SiC powder was β-Rundum Ultra Fine (an SiC powder composed ofgrains 0.3 μm in average particle size; a product of Ibiden Co., Ltd.).

Seven powder mixtures were prepared by adding the thus classified TiB₂powder and fine SiC powder with respect to alumina, at an amount byvolume as shown in Table 3. Each of the powder mixtures thus obtainedwas subjected to conventional ball milling technique in the pure ethanolwith alumina balls for 24 hours. The slurry thus obtained wassufficiently dried, and was crushed by dry ball milling technique for 12hours again. Thus was obtained the starting powder.

Then, in a similar manner as those in the foregoing Examples andComparative Examples, about 48 g each of the resulting powder mixtureswas charged into a graphite die (60 mm in inner diameter), and aftersubjecting the mixtures to precompression applied pressure of 10 MPa,the resulting compact was sintered using Frequency induction heating hotpressing apparatus (manufactured by Fuji Microwave Co., Ltd.) attemperatures shown in Table 3. The hot pressing was conducted by firstelevating the temperature to a predetermined sintering temperature, andthen maintaining the temperature constant for an hour in Argon gasatmosphere. The pressure applied to the molding was 30 MPa.

The resulting sintered bodies of ceramic composites were each cut intotest rectangular pieces each using a diamond cutter, and the four facesthereof were polished using a diamond wheel to obtain a surfaceroughness of #1000. By thus machining, each of the test specimens werefinished to a specified dimension of 3×4×36 mm, according to the JISR1601 standard three-point bending test. The bending strength wasmeasured on those test pieces at a span length of 30 mm and load wasapplied at a crosshead speed of 0.5 mm/minute. The fracture toughnesswas measured by an IF (Indentation Fracture) method with indenter loadsof 5 kg for a retention time of 10 seconds. The results are given inTable 3.

                  TABLE 3                                                         ______________________________________                                        Bending strength for Al.sub.2 O.sub.3 --TiB.sub.2 --SiC composites            varying in TiB.sub.2 /SiC addition                                                  Blending ratio                                                                              Sintering Bending                                                                              Fracture                                 Sample                                                                              (vol. %)      Tempera-  Strength                                                                             Toughness                                Nos.  Al.sub.2 O.sub.3                                                                      TiB.sub.2                                                                            SiC  ture (°C.)                                                                     (MPa)  (MPam.sup.1/2)                       ______________________________________                                        Comp. 100      0      0   1400     400   3.8                                  No. 1                                                                         Ex. 2 80      20      0   1600     950   5.8                                  Ex. 5 70      20     10   1700    1220   7.2                                  Ex. 6 65      30      5   1800    1180   7.3                                  Ex. 7 70      15     15   1750    1150   6.8                                  Ex. 8 70      10     20   1750    1100   6.7                                  Comp. 80       0     20   1700     950   5.2                                  Ex. 4                                                                         ______________________________________                                    

It can be seen clearly from Table 3 that the ceramic compositesaccording to the second embodiment of the present invention exhibits abending strength about three times as large as and a fracture strengthabout twice as large as those of the monolithic alumina ceramics. It canbe seen, moreover, that the bending strength and the fracture toughnessof the ceramic composites according to the present invention are bothhigher than those of the binary Al₂ O₃ -TiB₂ and Al₂ O₃ -SiC systems.

The fracture surface obtained after the measurements of bending strengthand fracture toughness were observed. The cracks were observed topropagate along a very complicated path due to the TiB₂ and SiCparticles having dispersed in the grains. Furthermore, the fracturesurface was understood to occur as an intergranular fracture.

Accordingly, it can be understood that the fracture toughness of theceramic composites according to the present invention is improved byfine TiB₂ and SiC particles having dispersed within the grains of thematrix, which deflect the cracks and the like. The bending strength inthis case is increased by the finely divide texture and the improvementis seen in fracture toughness.

As explained in detail in the foregoing, the ceramic compositesaccording to the present invention consist of a dense and finestructure, and is thereby significantly improved in mechanicalproperties such as bending strength. Accordingly, the ceramic compositesaccording to the present invention is particularly suited for use asheat resistant materials, cutting tool materials, wear-resistantmembers, refractories, and as structural materials having excellentresistance against thermal shock.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A ceramic composites consisting essentially of:apolycrystalline alumina matrix having a grain size of from 0.5 to 10 μm,fine particles of TiB₂, and fine powder of SiC, said fine particles ofTiB₂ and said powder of SiC being 2 μm or less in diameter and beingdispersed within said alumina grains, said ceramic composites containingfrom 5 to 30% by volume of TiB₂, from 5 to 30% by volume of SiC, andfrom 60 to 90% by volume of alumina.
 2. The ceramic composite as claimedin claim 1, wherein the content of fine particles of TiB₂ and SiC intotal is from 10 to 40% by volume.
 3. A process for manufacturingcomposite ceramics, which comprises:mixing materials consistingessential of from 5 to 30% by volume each of fine TiB₂ particles 2 μm orless in diameter and fine SiC powder 2 μm or less in diameter with from40 to 90% by volume of fine alumina particles 5 μm or less in diameter;molding the resulting powder mixture; and sintering the molding thusobtained at a temperature not lower than 1500° C.
 4. The process formanufacturing ceramic composites as claimed in claim 3, wherein, fineTiB₂ particles are initially added and mixed with fine-grained aluminaat first, and fine SiC powder are added subsequently to the mixture. 5.The ceramic composite as claimed in claim 1 wherein said ceramiccomposite has bending strength between 1100 and 1220 MPa, and fracturetoughness between 6.7 and 7.2 MPam^(1/2).
 6. The ceramic composite asclaimed in claim 1, wherein said TiB₂ particles have a diameter of 0.6micro meters or less, and said SiC powder has an average diameter of 0.3micro meters.
 7. The process for manufacturing ceramic composite asclaimed in claim 4, wherein said ceramic composite has bending strengthbetween 1100 and 1220 MPa, and fracture toughness between 6.7 and 7.2MPam^(1/2).
 8. The process for manufacturing ceramic composite asclaimed in claim 4, wherein said TiB₂ particles have a diameter of 0.6micro meters or less, and said SiC powder has an average diameter of 0.3micro meters.